License-Assisted Access via LTE (LAA-LTE) has recently been proposed as a technology to enable the cooperation of 3rd-Generation Partnership Project (3GPP) Long Term Evolution (LTE) systems on licensed carriers with unlicensed communication systems, such as wireless local area network (WLAN) communications systems.
IEEE 802.11 is a set of Media Access Control (MAC) and Physical layer (PHY) specifications for implementing WLAN computer communication in the 2.4, 3.6, 5 and 60 GHz frequency bands. The specifications are created and maintained by the IEEE Standards Committee IEEE 802. The base version of the standard was released in 1997 and has had subsequent amendments. The standard and amendments provide the basis for wireless network products using Wi-Fi.
A wireless local area network (WLAN) links two or more devices using a wireless distribution method, and may also provide a connection through an access point to another network. This gives users the ability to move around within a local coverage area and still be connected to the network. All devices that can connect to the WLAN are referred to as stations. Wireless stations fall into one of two categories: access points and clients. Access points (AP), normally routers, are base stations for the wireless network. They transmit and receive signals at radio frequencies for wireless enabled devices. Wireless clients can be mobile devices, such as laptops, personal digital assistants, IP phones and other smartphones, or fixed devices, such as desktops and workstations, that are equipped with a wireless network interface. The IEEE 802.11 standard has two basic modes of operation: an ad hoc mode and an infrastructure mode. In the ad hoc mode clients communicate directly peer-to-peer. In the infrastructure mode, clients communicate through an AP that serves as a bridge to other networks, such as the Internet or a Local Area Network (LAN) or wide area network (WAN). The following sections summarize some IEEE 802.11 characteristics.
Wi-Fi systems based on the IEEE 802.11 standards have many aspects in common with cellular systems in that they both provide orderly access to a shared wireless medium. One difference is the MAC protocol, which for cellular systems typically is scheduled, and for Wi-Fi is contention-based. This means that a receiving station does not know in advance what transmitting station it will receive data from and what transmission format that is used. The IEEE 802.11 MAC protocol is described in some more detail below.
The basic IEEE 802.11 MAC, the so-called Distributed Coordination Function (DCF), employs a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)-based protocol. The same protocol is applied by all stations, including the APs, i.e. in both downlink and uplink transmissions. The standard also supports a Point Coordination Function (PCF) mode, in which APs have more control over the medium usage. Supporting the PCF mode is however optional, and rarely implemented.
As depicted in FIG. 1, a station using the DCF mode (User A) and wishing to transmit a frame first senses the medium. If the medium is sensed to be idle for a certain minimum time, i.e. a so-called Distributed Inter Frame Space (DIFS), the frame is transmitted. The DIFS is 50 μs in the release IEEE 802.11b. If the medium is busy, as it is for user C in FIG. 1, the station first waits until the medium is sensed idle (defer). When this occurs, the station defers the transmission during a DIFS. As an immediate transmission after the expiration of the DIFS may lead to collisions if more than one station is waiting to transmit data, each transmitting station sets a back-off timer to a random delay, and transmits only when this back-off timer has expired instead of transmitting immediately at the expiration of the DIFS. The back-off timer is only activated when the medium is sensed idle. Whenever the medium is sensed busy, a deferral state is entered in which the back-off timer is not activated. When the back-off timer expires, the frame is transmitted. If the frame is successfully received by a station, the receiving station responds with an acknowledgement to the transmitting station. The acknowledgement is sent a Short Inter Frame Space (SIFS) after the data frame is received. The SIFS is 10 μs in the release IEEE 802.11b. Since a SIFS is shorter than a DIFS, no other station will access the medium during this time. If no acknowledgement is received by the transmitting station, the transmitting station generates a new back-off timer value, and retransmits the frame when the new back-off timer has expired. The reason for not receiving any acknowledgement may be either because the transmitted frame is lost, with the result that no acknowledgement is returned, or because the acknowledgement itself is lost. Even if the frame is successfully acknowledged, the transmitting station must generate a back-off timer value and wait for it to expire before transmitting the next frame. To avoid congestion when collisions occur, back-off timer values are drawn from distributions with larger and larger expected values for every retransmission attempt. For the nth transmission attempt, the back-off timer value is drawn from the uniform distribution U[0,min((CWmin)*2n−1−1, CWmax)]. CWmin and CWmax are constants with values that depend on the physical layer. For the release IEEE 802.11b the values are CWmin=31 and CWmax=1023. The back-off timer value is measured in units of slot times, which for release IEEE 802.11b are 20 μs long.
In the Enhanced DCF mode, defined in the release IEEE 802.11e standard, service prioritization is introduced. This is done by using back-off and deferral parameters that depend on a service type.
Since frames are transmitted after a DIFS when the medium is free, the minimum delay is equal to the transmission time plus a DIFS, which for release IEEE 802.11b is about 1 ms for a 1500 byte frame. The almost immediate acknowledgement, with a transmission time of around 0.1 ms, means that the Round Trip Time (RTT) on layer 2 may be of the order of 1 ms.
Because of the back-off and deferral times between transmissions, the medium is not fully used even at high traffic loads. The maximum link utilization reached depends on the frame size, and varies between 50% for voice to 70-80% for data.
The approaches described in the Background section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in the Background section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in the Background section.